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THE OPEN-HEARTH FURNACE.                             141
(3)  It is possible to make the back wall, in either type, by tilting the furnace to its extreme position and throwing bottom material on the back side, for this wall, which is nearly vertical during the regular operation, becomes more nearly horizontal when tipped over.
In the foregoing points both tilting types.share, but the original furnace has certain important advantages.
(4)   The back wall can be made more readily in the Campbell type, for in the Wellman construction no gas can be kept on the furnace when it is tipped, while in the first construction a flame is kept constantly going through.   The setting of a sand bottom requires an extremely high temperature, and it would be impossible to set sand on the back wall without raising the furnace to its full temperature.    It would, therefore, be impossible to do this in a AVellman furnace, while it has. been done regularly at Steelton.   In a basic furnace the Wellman furnace is able to coke and harden a tar mixture in place by the heat of the walls and bottom, but the work must be less satisfactory than in a furnace where the flame can immediately be put upon the dolomite and the coking be done quickly, and the furnace be heated for the next charge, instead of being cooled by exposure.
. (5) Owing to the ability to build the back wall in this manner a steep slope can be maintained, much steeper than can be kept in a stationary furnace. If a vertical wall could be maintained at the slag line, the action would be reduced to a minimum, -.because it would be impossible for pieces of ore or scrap to lodge anywhere, and because the area of the surface exposed to slag would be less.
(6)- The wear on the front or charging side is the on any other furnace, and there is the same liability to form holes along the slag line, but in the Campbell type such a hole is seldom a serious matter, for while the charge is in the furnace, and without interrupting the operation, the hearth may be tilted, the hole drained dry, filled with bottom material and set in the usual manner, after which the furnace may be returned to its proper position with practically a new bottom. Such repairs would be impossible with the Wellman type.
(7) The most important advantages arising from the ability to tip the furnace-without altering the flame comes in the use of